In recent years governments of industrialised countries have become increasingly concerned with control of pollution of the atmosphere by exhaust gases from engines and machinery, in particular from internal combustion engines of automobiles. Car manufacturers are commonly required to impose stringent tests on the pollution content of exhaust gases, and usually this is carried out by the manufacturer at his factory. There are available numerous types of measuring equipment for testing the pollution content of automobile exhaust gases and the type and standards of the pollution measuring equipment are set by the government concerned. Often the cars will be imported into the country setting the pollution standards, and for this and other reasons governments are becoming increasingly concerned with the maintenance of the pollution measuring equipment itself to the required standards of accuracy.
There has therefore grown up a requirement for equipment to test the pollution measuring equipment, and this is usually done by passing through the pollution measuring equipment a gas comprising a standardized pollution gas of a specified strength and composition, combined with and diluted by a relatively inert gas which does not affect the pollution measuring equipment. Commonly the active constituent gas consists of NO.sub.2 and the diluent gas consists of air. The test is carried out by mixing the active gas and the diluent in a series of different proportions, and feeding the resultant mixtures in sequence through the pollution testing equipment and reading the results from the testing equipment. The recorded results are then compared with the known proportions of the test gases, to determine whether the required linearity has been maintained by the pollution testing equipment.
The present invention is concerned, in a particular aspect with the problem of preparing the required test gases in varying concentrations. In known arrangements, the active constituent is added to the diluent in required proportions by volume by varying the rates of flow of the two flows of gas. Commonly one of the gases is set at a constant flow, and the rate of the other gas is increased or decreased as required to vary the proportions. The disadvantage of this arrangement is that the resultant combined flow of the two gases varies in its total rate of flow, and such variation in rate of flow of the combined test gas prevents accurate checking of the pollution testing equipment. Where an attempt has been made to provide a constant rate of flow for the combined test gas, this has involved the venting to atmosphere a proportion of the combined test gas. Since the gases involved in the testing are commonly toxic, this venting to atmosphere of unwanted pollution gases has proved a disadvantage of previous arrangements for checking pollution testing equipment. A further disadvantage of previous testing methods has been that the venting to atmosphere of varying amounts of the active constituent and/or diluent gas of the test gases has been wasteful of the gases used. Since the preparation of these standardized gases is necessarily carried out with considerable accuracy, waste of the gases used can prove unnecessarily expensive.